1. Field of the Invention
The present invention relates to toners for developing electrostatic images in electrophotography, electrostatic recording and electrostatic printing, and for forming images in toner jet systems.
2. Description of the Related Art
Development of images formed on photoconductive materials by electrostatic means with toners is well known, as disclosed in, for example, U.S. Pat. No. 2,297,691 and Japanese Patent Publication Nos. 42-23910 and 43-24748. In such development, an electrostatic latent image is formed on a photosensitive member by various means, and a finely pulverized material responsive to electrical charge, called a "toner" is deposited onto the latent image to form a toner image corresponding to the electrostatic latent image.
The toner is transferred onto a surface of an image-holding member such as paper, if necessary, and then is fixed by heat, pressure, or vapor of a solvent to produce a copy. When the toner image is transferred, the process is generally provided with a step for removing the residual toner.
Disclosed methods for visualizing electrostatic latent images using toners include a powder cloud method disclosed in U.S. Pat. No. 2,221,776, a cascade developing method disclosed in U.S. Pat. No. 2,618,552, a magnetic brush method disclosed in U.S. Pat. No. 2,874,063, and a method using an electronically conductive, magnetically attractable toner disclosed in U.S. Pat. No. 3,909,258.
A typical toner used in these developing methods is composed of a particulate resin containing a colorant, and is prepared by mixing a thermoplastic resin with the colorant and then pulverizing the mixture. Thermoplastic resins which are most commonly used are polystyrene resins. Examples of other usable thermoplastic resins include polyester resins, epoxy resins, acrylic resins, and urethane resins. A typical colorant is carbon black. In magnetic toners, iron oxide-based black magnetic powder is widely used. In a two-component developer system, a toner is generally used in admixture with carrier particles, such as glass beads, powdered iron, or powdered ferrite.
Recently, monochrome copying machines have rapidly been replaced by full-color copying machines, and two-tone and full-color copying machines have been used. Improvements in reproducibility of color and gradation are reported in, for example, Journal of Electrophotographic Society, Vol. 22, No. 1, (1983); and ibid. Vol. 25, No. 1, 52(1986).
These full-color electrophotographic images used in practice, however, are not comparable to the originals and do not always satisfy those who are familiar with beautifully processed color images, such as on television, in photographs, and in color prints.
Formation of a color image by a full-color electrophotographic method generally includes color reproduction using three primary colors including yellow, magenta, and cyan. In this method, a photoconductive layer is exposed to light from an original which passes through a color-decomposing filter, which color is complementary to the color of toner so that an electrostatic image is formed on the layer. Next, a toner is held on a recording medium by developing and transferring steps. These steps are repeated several times so that different toners overlap on the same recording medium while adjusting the registration, and then only one fixing step is performed to fix a full-color image.
In a two-component development system using a developer composed of a toner and a carrier, the toner in the developer is charged to a predetermined polarity and a predetermined amount of electricity by friction with the carrier so that an electrostatic attractive force develops an electrostatic image. Thus, a satisfactory visible image is obtained when the toner has sufficient triboelectric charging characteristics which are determined by the interaction with the carrier.
Much research has been performed regarding various materials for developers so as to achieve sufficient triboelectric charging characteristics. The scope of the research includes development of carrier core materials, carrier coating agents, optimization of the amount of the coating agent, development of charge controlling agents and flowability-imparting agents added to toners, and improvements in binders as base materials.
For example, charging auxiliaries, such as chargeable fine particles, are added to toners as developers for the purpose of stabilizing triboelectric characteristics. Examples of such charging auxiliaries include powdered resins having an opposite polarity to toners, as disclosed in Japanese Patent Publication No. 52-32256 and Japanese Patent Application Laid-Open No. 56-64352; and a fluorine-containing compound, as disclosed in Japanese Patent Application Laid-Open No. 61-160760. Development of various charging auxiliaries continues.
Various methods for adding the above-mentioned charging auxiliaries have also been proposed. A typical method is coating of a charging auxiliary onto the surface of the toner particle by an electrostatic force or van der Waals attraction between the toner particle and the charging auxiliary caused during stirring or mixing. In such a method, however, the surface of the toner particle is not satisfactorily coated with the additive, and free additive particles which do not adhere to the toner unavoidably occur and form agglomerates. The larger the specific resistance of the charging auxiliary or the finer the particle diameter, the more significant this tendency. As a result, the toner has insufficient and unstable charging characteristics and results in, images for example, with nonuniform image density and fog. Furthermore, the charging auxiliary content may change during a continuous copying operation so that the initial image quality is not maintained.
In another method, a charging auxiliary, a binding resin and a colorant are added during production of a toner. The charging auxiliary is, however, barely homogenized. Furthermore, only the charging auxiliary which is present in the vicinity of the toner particle surface contributes to charging, and thus the charging auxiliary which is present in the interior of the particle does not contribute to charging. Thus, it is difficult to control the amount of the charging auxiliary to be added and the amount of the charging auxiliary adhered onto the toner particle surface. The toner obtained by such a method has unstable triboelectric charging characteristics, and thus does not have satisfactory developing characteristics. Accordingly, mere use of the charging auxiliary does not result in a toner having satisfactory quality.
Stabilization of triboelectric charging characteristics by adding external additives has been proposed. For example, Japanese Patent Application Laid-Open Nos. 61-275862, and 61-275863 disclose use of hydrophobic particulate alumina. This particulate alumina is coated with an amino-modified silicone oil, which inevitably agglomerates during hydrophobic treatment. As a result, the toner does not have high flowability.
Hydrophobic particulate alumina is also disclosed in Japanese Patent Application Laid-Open Nos. 62-8164, 62-129860, 62-129866, 62-209538, 4-345168, and 4-345169. These patent applications, however, do not suggest that important factors for achieving uniform hydrophobic treatment include the reactivity of particulate alumina with a hydrophobic agent and the crystal structure of the particulate alumina. The particulate alumina is primarily used for stabilization of charging, because silica is also added in order to impart high flowability to the toner. Providing high flowability and abrasive ability by alumina itself are open to further improvement.
Furthermore, Japanese Patent Application Laid-Open No. 2-251970 discloses an external alumina lubricant, the surface of which is treated with a coupling agent. However, such treatment of conventional alumina does not sufficiently stabilize charging characteristics under high temperature, high-humidity environments, and does not impart satisfactory flowability to the toner.
For the purpose of providing sufficient flowability, stabilized charging, and particularly prevention of overcharging in low-temperature, low-humidity environments, Japanese Patent Application Laid-Open Nos. 4-280254, 4-280255, and 4-345169 disclose fine alumina particle having a hydrophobicity of 40% or more. Although such alumina is effective in stabilization of charging characteristics, it is still unsatisfactory in terms of flowability compared to fine particulate materials having high BET specific surface areas, such as silica. Thus, a fine particulate hydrophobic alumina, which is homogenized, which does not contain large amounts of agglomerates, and which has a high BET specific surface area, is still desired.
Japanese Patent Application Laid-Open No. 3-191363 discloses a toner containing hydrophobic abrasive .gamma.-alumina in order to achieve uniform abrasive effects of alumina when an amorphous silicon photosensitive member is used. Thus, a toner containing such alumina does not simultaneously satisfy flowability and stabilized charging characteristics, although it has abrasive characteristics.
Currently, high definition and high quality of images are commercially required in copying machines. Achievement of high quality color images has been attempted using toners having smaller particle diameters. A finer toner having a larger specific surface area usually carries increased amounts of charge; hence, the image density may decrease and durability will deteriorate. Furthermore, highly charged toner particles have high bonding force, resulting in decreased flowability. As a result, toner supply is not stabilized and the supplied toner has insufficient triboelectric characteristics.
Since color toners do not contain magnetic substances and conductive substances such as carbon black, charges tend to increase without discharging. This phenomenon is particularly noticeable when a polyester binder having high charging characteristics is used.
The following are characteristics required for color toners:
(1) The fixed toner must almost completely melt to an extent in which the shape of toner particles are not recognized in order to prevent random scattering of light which decreases color reproducibility. PA1 (2) Each color toner must have transparency so that an underlying different color toner layer is visible. PA1 (3) Each color toner must have balanced hue and reflective characteristics, and have high color saturation.
Many binding resins have been researched to develop a toner satisfying the above-mentioned requirements. Polyester resins have been generally used as binding resins for color toners. Since temperature and humidity affect polyester resin-containing toners, these toners tend to be excessively charged in low-humidity environments, but are usually insufficiently charged in high-humidity environments. Accordingly, development of color toners having stabilized amounts of charges in various environments has been desired